The basics
Light and matter (anything made of atoms or the particles that make up an atom) are not supposed to mix. But scientists have now found a way to combine light and matter (electrons in this case) to make a new object called the exciton-polariton, whose weird quantum properties may enable us to develop electronic technologies that use a lot less energy, help build the quantum computer, detect black holes and stars colliding and build new types of low-energy lasers.
Going in depth
What follows are details aimed at students in year 10-12, and any interested adults who want to dive a bit deeper into the physics of exciton-polaritons and what they might mean for society’s future.
With exciton-polaritons we are talking about the creation of entirely new states of matter. What does this mean?
The exciton-polariton is a type of quasiparticle that is a hybrid of light and matter. It exists in that weird superposition state that means it is both light and matter at the same time, and it has some intriguing properties. One of these properties is a superfluid-like state where exciton-polaritons do not randomly move about and bumping into each other like atoms or particles in a normal system – even a solid, but they flow or move as one single large exciton polariton; there is no friction and importantly for FLEET researchers, no electrical resistance. The superfluid property could see it applied in the next generation of computer chips to enable low energy-electronics. It also has potential to help build the quantum computers and quantum memory devices. It is already being used to construct extremely low-energy lasers (very concentrated sources of light that can travel long distances), and to create exotic forms of light called squeezed light, for example to sense extremely small magnetic and gravitational fields that help us study stuff from the atomically small to the astronomically large such as the collision of stars and blackholes.
Parth G provides a nice description on what squeezed light is
SciShow give you a typically entertaining insight into Superfluids
But we are still in the early stages of understanding and learning about exciton-polaritons and the potential of these strange quasiparticles is still being worked out. It is now a massive field of research that includes materials engineers, chemists and physicists. FLEET is one of the main research groups in Australia working in this space with an aim to use the technology to help develop low-energy electronics.
Check out some of FLEET’s research stories about exciton-polaritons
Making an exciton-polariton
Physicists use mirrors and specially constructed 2D materials* to enable exciton-polaritons to exist. If you could hold exciton-polaritons in your hands and actually see one (which you can’t) you couldn’t tell if you had a ball of light or a piece of solid matter in your hand. What you would have is that weird superposition of neither light nor matter – or light and matter at the same time. That is, it would be morphing from light-solid-light-solid-light-solid so fast it is impossible to detect which state they are in at any point in time…Ahh, Schrodinger.
*2D materials are materials that are one atom thick. Graphene is an example of a 2D material and was the first one scientist created. There is now a zoo of advanced 2D materials that scientists have developed, each with unique properties that scientists hope to use or have used to create advanced materials that can transform our ability to create and build everything from more efficient solar panels, batteries, to lighter, stronger, greener building materials or, in FLEET’s case, low-energy electronics.
Let us break the exciton-polariton down a bit.
Matter
We know that matter (whether solid, liquid or gas) is made up of atoms that consist of a nucleus where the protons and neutrons sit. Whizzing around the nucleus is a cloud of charged particles called electrons. [Link to FLEET schools atom section]
Light
In the 1900s Max Planck suggested that light was also made of small packets (or tiny particles) called photons that travel at about 300,000 kilometers per second (ie, light speed). Light contains energy and it can push charged particles around and knock them from the place in the atom.
Parth does it again with a beginner’s explanation of Maxwell’s equation
Exciton polaritons
When certain specially developed atomically thin (2D) materials are placed in front of light, a photons that will hit and stick to the electron in the material, knocking them out from their place around the atom. The now free electron leaves a “hole” in the material, which just means the atom is now more positive because it has one less negative particle (electron). The free negative electron is attracted to that hole (positive and negative attract) and will find its way back to the hole and release the photon which then zooms off again.
The clever bit
If you place two mirrors close together on either side of your 2D material, you can trap the photon in between them by making it bounce back and forth between the mirrors. Then something weird happens. The photon, now bouncing between the mirrors, will kick out the electron forming a hole in the 2D material. The electron then finds its way back to the hole and falls in, emitting the photon that, instead of now zooming off, will bounces back and forth between material and mirror as it continually kicks out electrons that fall back to the hole and re-emit the photon. This continual process takes place so quickly that if you were to observe it you can’t distinguish whether you are looking at the photon of electron-electron hole. This new object formed by the mixing of light (photons) and matter (electron-hole pair) is called the exciton-polariton. See Figure 1 below.
Figure 1. When a photon passes through the one-way mirror, it can kick an electron from its low energy state into a higher energy state (the conductance band) where it can become mobile and generate a current. This leaves a “hole” which is another way of saying that atom in the 2D material is now more positive. The negative electron will be attracted to the positive “hole” and recombine with the material, and kick the photon out again. But the photon, which is now trapped in between the mirror and 2D material, will kick the electron out again and repeat the process over and over. Note we have not perfected the prefect mirror trap yet. Some photons will inevitably leak back through the mirror to the outside. This leakage is actually what enable us to measure what is happening in the system. (Image by Sangeet Kumar)
Semiconductors
The special atomically thin materials used are called semiconductors. They have properties in between conductors and insulators. A conductor can conduct electricity automatically and easily, and insulator cannot conduct electricity at all. A semiconductor is a material that has the ability to conduct electricity, but it needs some energy added to it, or something done to it to make it happen.
A semi-conductor will have two ranges or bands of energy that the electrons can have, the lower energy band called the valency band and the higher energy band called the conduction band. Only the electrons in the conduction band of the semiconductor help in conducting electricity. Electrons will preferentially first fill in the lower energy (valency) band of semiconductors, unless you knock them with enough energy to kick them into the conduction band, which can come in the form of heat energy or light (a photon). Once the electron is kicked from the valency band to the conduction band it becomes free to conduct electricity and you get left with your “hole” in the sea of electrons that fill the valency band. This hole in the valency band behaves as a positively charged particle with the opposite charge of the negative electron removed from the electron sea. The positively charged hole in the valency band and the negatively charged electron in the conduction band attract each other due to the Coulomb force and form a Hydrogen-like atom called the exciton. When the photon joins in as described above, you get your exciton-polariton.
The mirrors – optical microcavity
Optical microcavity is the scientific name of the two tiny mirrors that are used to trap the photons. The mirrors are made to be reflective in only one direction. That is, light will pass through from one side of the mirror, but be reflected on the other side, just like the windows you see in TV police shows where the suspects being interviewed by the police can’t see who is looking in on them from the other side of the window.
Acknowledgement. This article was written by FLEET’s Sangeet Kumar (PhD Student)
Back to Electricity, Conductors and Insulators or FLEET Schools